1. Field of the Invention
This invention relates to an antiskid brake system for a vehicle.
2. Description of the Prior Art
In an antiskid brake system for a vehicle, the braking pressures applied to the wheels of the vehicle in response to depression of the brake pedal are controlled so that the wheel speeds are reduced at a target deceleration or at a target slip ratio, thereby preventing lock and skid of the wheels and stopping the vehicle in a short distance with a directional stability. Such an antiskid control is generally initiated when the deceleration of the wheel speed exceeds a predetermined threshold value or the wheel speed falls greatly relative to the vehicle speed.
There have been proposed various antiskid brake systems. For example, in the antiskid brake system disclosed in Japanese Unexamined Patent Publication No. 60(1985)-1061, the braking pressures are reduced as the steering angle increases, thereby improving both the braking efficiency and the driving stability.
When the brake pedal is sharply pushed down, the wheel speed falls at a large deceleration relative to the vehicle until the slip ratio of the wheel reaches a certain value, and then reduction in the wheel speed once slows down. When the brake pedal is further kept depressed, the wheel speed further falls. This is due to the fact that the friction coefficient between the wheel and the road surface becomes maximum when the slip ratio of the wheel reaches 10 to 20%. (See FIG. 2) That is, until the slip ratio becomes 10 to 20%, the wheel speed sharply falls due to small friction and then reduction in the wheel speed once slows down due to large friction when the slip ratio becomes 10 to 20%.
At the time when the wheel speed once slows down, a maximum braking force acts on the wheel and accordingly the antiskid control is not required yet. If the antiskid control is initiated at the time, the braking pressure applied to the wheel is reduced, whereby the slip ratio is reduced and the friction between the wheel and the road surface is reduced. That is, preferably the antiskid control is initiated when the slip ratio is about to increase (i.e., when the friction between the wheel and the road surface is about to decrease).
However, the deceleration of the wheel speed is generally as large as 2.5 G until the slip ratio of the wheel reaches 10 to 20%. Accordingly, in order to prevent the antiskid control until the slip ratio of the wheel reaches 10 to 20%, the threshold value must be as large as, for instance, 3G. However this gives rise to a problem that when the wheel speed further falls after reduction in the wheel speed once slows down, that is, when the antiskid control comes to be required, the antiskid control can be initiated late.
That is, when the threshold value for initiation of the antiskid control is set large, the antiskid control cannot be quickly initiated though premature initiation of the antiskid control in response to sharp depression of the brake pedal can be avoided, and when the threshold value for initiation of the antiskid control is set small, premature initiation of the antiskid control in response to sharp depression of the brake pedal is apt to occur though the antiskid control cannot be quickly initiated.